Fix ADOdb errors when creating database with Oracle

ADOdb was triggering errors in 2 distinct cases:

• calling config_get() to retrieve the db_type was returning 'mysql'
  instead of 'oci8' due to caching of the value in a prior call to the
  function. Setting the global variable using config_set_global()
  rather than directly assigning it with $GLOBALS clears the cache and
  allows normal processing.

• when calling NewDataDictionary(), for an unknown reason an error
  occurs in adodb.inc.php line 4350 trying to include_once the oci8
  data dictionary class. This was resolved by using @ operator to
  ignore the error

Affects 0013227